Title: Petascale Simulations of the Morphology and the Molecular Interface of Bulk Heterojunctions

Abstract

Understanding how additives interact and segregate within bulk heterojunction (BHJ) thin films is critical for exercising control over structure at multiple length scales and delivering improvements in photovoltaic performance. The morphological evolution of poly(3-hexylthiophene) (P3HT) and phenyl-C 61-butyric acid methyl ester (PCBM) blends that are commensurate with the size of a BHJ thin film is examined using petascale coarse-grained molecular dynamics simulations. When comparing 2 component and 3 component systems containing short P3HT chains as additives undergoing thermal annealing we demonstrate that the short chains alter the morphol- ogy in apparently useful ways: They efficiently migrate to the P3HT/PCBM interface, increasing the P3HT domain size and interfacial area. Simulation results agree with depth profiles determined from neutron reflectometry measurements that reveal PCBM enrichment near substrate and air interfaces, but a decrease in that PCBM enrich- ment when a small amount of short P3HT chains are integrated into the BHJ blend. Atomistic simulations of the P3HT/PCBM blend interfaces show a non-monotonic dependence of the interfacial thickness as a function of number of repeat units in the oligomeric P3HT additive, and the thiophene rings orient parallel to the interfacial plane as they approach the PCBM domain. Using the nanoscale geometries of themore » P3HT oligomers, LUMO and HOMO energy levels calculated by density functional theory are found to be invariant across the donor/acceptor interface. Finally, these connections between additives, processing, and morphology at all length scales are generally useful for efforts to improve device performance.« less

@article{osti_1286951,
title = {Petascale Simulations of the Morphology and the Molecular Interface of Bulk Heterojunctions},
author = {Carrillo, Jan-Michael Y. and Seibers, Zach and Kumar, Rajeev and Matheson, Michael A. and Ankner, John F. and Goswami, Monojoy and Bhaskaran-Nair, Kiran and Shelton, William A. and Sumpter, Bobby G. and Kilbey, S. Michael},
abstractNote = {Understanding how additives interact and segregate within bulk heterojunction (BHJ) thin films is critical for exercising control over structure at multiple length scales and delivering improvements in photovoltaic performance. The morphological evolution of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blends that are commensurate with the size of a BHJ thin film is examined using petascale coarse-grained molecular dynamics simulations. When comparing 2 component and 3 component systems containing short P3HT chains as additives undergoing thermal annealing we demonstrate that the short chains alter the morphol- ogy in apparently useful ways: They efficiently migrate to the P3HT/PCBM interface, increasing the P3HT domain size and interfacial area. Simulation results agree with depth profiles determined from neutron reflectometry measurements that reveal PCBM enrichment near substrate and air interfaces, but a decrease in that PCBM enrich- ment when a small amount of short P3HT chains are integrated into the BHJ blend. Atomistic simulations of the P3HT/PCBM blend interfaces show a non-monotonic dependence of the interfacial thickness as a function of number of repeat units in the oligomeric P3HT additive, and the thiophene rings orient parallel to the interfacial plane as they approach the PCBM domain. Using the nanoscale geometries of the P3HT oligomers, LUMO and HOMO energy levels calculated by density functional theory are found to be invariant across the donor/acceptor interface. Finally, these connections between additives, processing, and morphology at all length scales are generally useful for efforts to improve device performance.},
doi = {10.1021/acsnano.6b03009},
journal = {ACS Nano},
number = 7,
volume = 10,
place = {United States},
year = {Thu Jul 14 00:00:00 EDT 2016},
month = {Thu Jul 14 00:00:00 EDT 2016}
}

Monovalent ions such as alkalis and halides play essential roles in aqueous and biological systems; thus a proper representation of these ions is significant in explicit molecular dynamics simulations. In this study, we re-parameterize ions interaction potentials based on TIP4PEw water at 298K with the additive non-polarizable models. The charges of the ions were kept as +1 for alkali metal ions (Li+, Na+, K+, Rb+, Cs+) and -1 for halides (F-, Cl-, Br-, I-). The experimental enthalpies of hydration were used to optimize the potential parameters. To validate the potential models, extensive molecular dynamics calculations were carried out to examinemore » the bulk, interfacial, static structural and dynamical properties of the aqueous ionic solutions. These included radial distribution functions, angle distributions, velocity autocorrelation functions, diffusion coefficients, binding energies, mean residence time, surface potential, and potential of mean force. The computed results agreed with the experimental data and observations. LXD was funded by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, US Department of Energy. Battelle operates the Pacific Northwest National Laboratory for the U.S. Department of Energy.« less

Organic photovoltaics (OPVs) are topic of extensive research for their potential application in solar cells. Recent work has led to the development of a coarse-grained model for studying poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) blends with molecular simulations. Here we provide further validation of the force field and use it to study the thermal annealing process of P3HT:PCBM blends. A key finding of our study is that, in contrast to a previous report, the annealing process does not converge at the short time scales reported. Rather, we find that the self assembly of the blends is characterizedmore » by three rate dependent stages that require much longer simulations to approach convergence. Using state-of-the- art high performance computing, we are able to study annealing at length and time scales commensurate with devices used in experiments. Our simulations show different phase segregated morphologies dependent on the P3HT chain length and PCBM volume fraction in the blend. For short chain lengths, we observed a smectic morphology containing alternate P3HT and PCBM domains. In contrast, a phase segregated morphology containing domains of P3HT and PCBM distributed randomly in space is found for longer chain lengths. Theoretical arguments justifying stabilization of these morphologies due to shape anisotropy of P3HT (rod-like) and PCBM (sphere-like) are presented. Furthermore, results on structure factor, miscibility of P3HT and PCBM, domain spacing and kinetics of phase segregation in the blends are presented in detail. Qualitative comparison of these results with small-angle neutron scattering experiments from literature is presented and an excellent agreement is found.« less

Here we present the impact of various polymeric compatibilizers, including end-functionalized P3HTs and diblock copolymers containing P3HT, on the structure and function of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunctions. Careful analyses of small angle neutron scattering curves provide a measure of the miscibility of PCBM in P3HT, the average PCBM domain size, and the interfacial area between PCBM and the P3HT-rich phase in the uncompatibilized and compatibilized systems. Differential scanning calorimetry (DSC) also provides information regarding the changes in the crystallinity of P3HT due to the presence of the compatibilizer. Results show that most compatibilizers cause the domainmore » sizes to decrease and the P3HT crystallinity to increase; however, some cause an increase in domain size, suggesting that they are not effective interfacial modifiers. The correlation of morphology with photovoltaic activity shows that the decreased domain size, increased crystallinity and increased interfacial area do not always result in improved power conversion efficiency (PCE). It appears that the introduction of an insulating molecule at the PCBM:P3HT interface as a compatibilizer results in a decrease in PCE. Thus, the presence of the compatibilizer at this interface dominates the photovoltaic activity, rather than the morphological control.« less

We show that despite very similar crystallographic properties and work function values in the bulk, epitaxial Fe and Cr metallizations on Nb:SrTiO3(001) generate completely different heterojunction electronic properties. Cr is Ohmic whereas Fe forms a Schottky barrier with a barrier height of 0.50 eV. This contrast arises because of differences in interface chemistry. In contrast to Cr [Chambers, S. A. et al., Adv. Mater. 2013, 25, 4001.], Fe exhibits a +2 oxidation state and occupies Ti sites in the perovskite lattice, resulting in negligible charge transfer to Ti, upward band bending, and Schottky barrier formation. The differences between Cr andmore » Fe are understood by performing first-principles calculations of the energetics of defect formation which corroborate the observed interface chemistry and structure.« less

We show that despite very similar crystallographic properties and work function values in the bulk, epitaxial Fe and Cr metallizations on Nb:SrTiO 3(001) generate completely different heterojunction electronic properties. Cr is Ohmic whereas Fe forms a Schottky barrier with a barrier height of 0.50 eV. This contrast arises because of differences in interface chemistry. In contrast to Cr [Chambers, S. A. et al., Adv. Mater. 2013, 25, 4001.], Fe exhibits a +2 oxidation state and occupies Ti sites in the perovskite lattice, resulting in negligible charge transfer to Ti, upward band bending, and Schottky barrier formation. The differences between Crmore » and Fe are understood by performing first-principles calculations of the energetics of defect formation which corroborate the observed interface chemistry and structure.« less